Photodetection and current control devices

ABSTRACT

A photodetection device and a current control device, both including means responsive to the intensity of incident radiation for providing an output signal corresponding thereto. The responsive means includes a thin semiconductor film comprising a solid amorphous silicon or amorphous germanium host matrix having electronic configurations and an energy gap. At least one compensating element is introduced into that matrix for reducing the density of states in the energy gap thereof.

Related Applications

This is a continuation of application Ser. No. 710,359 filed Apr. 18,1986 which is a continuation of application Ser. No. 540,153 filed Oct.7, 1983 which is a continuation of application Ser. No. 427,688 filedSept. 29, 1982, now U.S. Pat. No. 4,409,605, issued Oct. 11, 1983, whichis a continuation of application Ser. No. 222,489, filed Jan. 5, 1981,now abandoned, which is a continuation-in-part of application Ser. No.104,284 filed Dec. 17, 1979, now abandonded, which is a division ofapplication Ser. No. 887,353 filed Mar. 16, 1978, now U.S. Pat. No.4,226,898, issued Oct. 7, 1980.

BACKGROUND OF THE INVENTION

The invention relates to amorphous semiconductor bodies with uniquelylow defect states in the energy gap, such as dangling bonds,recombination centers, etc., to provide improved amorphous semiconductorfilms which have characteristics like those found in correspondingcrystalline semiconductors.

(By the term "amorphous" is meant a material which has long rangedisorder, although it may have an ordered structure in the short orintermediate range order, or it may contain isolated domains having anordered structure in a primarily amorphous matrix.)

The amorphous films involved have one of their most importantapplications in photovoltaic devices, and current control devices, suchas various p-n junction devices such as p-i-n and m-i-s, devices,rectifiers, transistors or the like, where heretofore crystallinesemiconductor bodies have been used in their fabrication.

The principles involved in the invention have their most important anduseful application to amorphous silicon and siliconalloy bodies,especially thin films of such materials, although many aspects thereofare also applicable to various other similar amorphous semiconductorbodies, and formed by elements including individual elements or mixturesor alloys of elements which have localized defect states in the energygap which adversely affect certain desired electrical characteristicsthereof.

Amorphous silicon-containing films, if made equivalent to crystallinesilicon films, would have many advantages over such crystalline siliconfilms (e.g. lower cost, larger area, easier and faster manufacture). Themain purpose of this invention is to provide amorphous semiconductorbodies which have characteristics resembling those of correspondingcrystalline materials.

When crystalline semiconductor technology reached a commercial state, itbecame the foundation of the present huge semiconductor devicemanufacturing industry. This was due to the ability of the scientist togrow substantially defect-free germanium and particularly siliconcrystals, and then turn them into extrinsic materials with p-type andn-type conductivity regions therein. This was accomplished by diffusinginto such crystalline material parts per million of donor (n) oracceptor (p) dopant materials introduced as substitutional impuritiesinto the substantially pure crystalline materials, to increase theirelectrical conductivity and to control their being either of a p or nconduction type. The fabrication processes for, making p-n junctioncrystals involve extremely complex, time consuming, and expensiveprocedures. Thus, these crystalline materials useful in solar cells andcurrent control deivces are produced under very carefully controlledconditions by growing individual single silicon or germanium crystals,and when p-n junctions are required, by doping such single crystals withextremely small and critical amounts of dopants.

These crystal growing processes produce such relatively small crystalsthat solar cells require the assembly of many single crystals toencompass the desired area of only a single solar cell panel. The amountof energy necessary to make a solar cell in this process, the limitationcaused by the size limitations of the silicon crystal, and the nenessityto cut up and assemble such a crystalline material have all resulted inan impossible economic barrier to the large scale use of crystallinesemiconductor solar cells for energy conversion. Further, crystallinesilicon as an indriect optical edge which results in poor lightabsorption in the material. Because of the poor light absorption,crystalline solar cells have to be at least 50 microns thick to absorbthe incident sunlight. Even if the single crystal material is replacedby polycrystalline silicon with cheaper production processes, theindirect optical edge is still maintained; hence the material thicknessis not reduced. The polycrystalline material also involves the additionof grain boundaries and other problem defects.

An additional shortcoming of the crystalline material, for solarapplications, is that the crystalline silicon band gap of about 1.5 eV.The admixture of germanium, while possible, further narrows the band gapwhich further decreases the solar conversion efficiency.

In summary, crystal silicon devices have fixed parameters which are notvariable as desired, require large amounts of material, are onlyproducible in relatively small areas and are expensive and timeconsuming to produce. Devices manufactured with amorphous silicon caneliminate these crystal silicon disadvantages. Amorphous silicon has anoptical absorption edge having properties similar to a direct gapsemiconductor and only a material thickness of one micron or less isnecessary to absorb the same amount of sunlight as the 50 micron thickcrystalline silicon. Further, amorphous silicon can be made faster,easier and in larger areas than can crystal silicon.

Accordingly, a considerable effort has been made to develop processesfor readily depositing amorphous semiconductor alloys or films, each ofwhich can encompass relatively large areas, if desired, limited only bythe size of the deposition equipment, and which could be readily dopedto form p-type and n-type materials where p-n junction devices are to bemade therefrom equivalent to those produced by their crystallinecounterparts. For many years such work was substantially unproductive.

W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics,University of Dundee, in Dundee, Scotland, did some work "SubstitutionalDoping of Amorphous Silicon", as reported in a paper published in SolidState Communications, Vol. 17, pp. 1193-1196, 1975. As there reported anamorphous silicon film was formed by glow discharge deposition of silane(SiH₄) gas in the presence of a doping gas of phosphine (PH₃) for an-type conduction or a gas of diborane (B₆ H₆) for p-type conduction.These gases were premixed and passed through a reaction tube where thegaseous mixture was decomposed by an r.f. glow discharge and depositedon a substrate at a high substrate temperature of about 350°-600° K. Thematerial so deposited on the substrate was an amorphous silicon hostmatrix material where the phosphorous or boron formed dopants in thesilicon host matrix material on concentrations between about 5×10⁻⁶ and10⁻² parts per volume. When the localized defect states in the energygap of the undoped form of these materials were measured it was foundthat the density of localized defect states in the energy gap thereofwas substantially reduced from that previously measured for otheramorphous silicon films deposited by other processes. However, a muchmore substantial reduction in the minimum density of these localizeddefect states was necessary to bring the electrical characteristics ofamorphous silicon materials much closer to those of correspondingcrystalline materials.

It is believed that it was not originally known by these researchersthat the silicon films they deposited were, instead of relatively pureamorphous silicon films, a composition of silicon and hydrogen and thatthe hydrogen combined with the silicon, to eliminate many of thelocalized defect states. However, after this initial development of theglow discharge deposition of silicon from silane gas was carried out,work was done by others on the sputter depositing of amorphous siliconfilms in an atmosphere of mixture or argon (required by the sputteringdeposition process) and molecular hydrogen, to determine the results ofsuch molecular hydrogen on the characteristics of the depositedamorphous silicon film. This research indicated that the hydrogen actedas a compensating agent to reduce the localized defect states of theenergy gap. However, the degree of reduction in the density of localizeddefect states achieved by this sputter deposition process was less thanthat achieved by the silane deposition process described above (as wouldbe expected since sputter and evaporation deposition processesinherently produce amorphous films with much higher densities oflocalized states than does a glow discharge deposition process). This isthe reason that it ws not believed that sputter or evaporationdeposition processes could successfully produce amorphous semiconductorfilms functionally equivalent to similar crystalline materials used insolar cell and current control devices. Also, the sputtering processmust be carried out under certain critical partial pressure limitations,and since such partial pressures are effected both by the amount ofargon and hydrogen gas present, the amount of molecular hydrogen gaswhich could be introduced into the sputtering atmosphere was accordinglylimited.

The amount of a compensating material like hydrogen theoretically neededto eliminate the dangling bonds in amorphous semiconductor materials isonly a small portion of one atomic precent thereof. Subsequently, it wasdetermined that hydrogen compensated some of the dangling bonds ofamorphous silicon materials, but the atomic percentage of hydrogen whichcombined with the silicon was found to be in alloying percentages, whichare amounts at least about 1 to 5 percent. Whether such alloying amountsof hydrogen was beneficial or not to the amorphous silicon composition,to our knowledge, has not been commented upon in the publishedliterature or otherwise.

The difficulty encountered heretofore in reducing the density oflocalized defect states in the energy gap of amorphous semiconductormaterials like silicon and others to desirably low levels, so that thesematerials are more nearly equivalent of corresponding crystallinematerials, is believed to be explainable in the following manner. At ornear the Fermi level of these materials deposited, for example, by theglow discharge of silane, are two bumps of relatively high densitystates in the energy gap which are apparently related to the remainingdangling bond density. They are located substantially at about 0.4 eVbelow the conduction band E_(c) and above the valence band E_(v). Whenthe glow discharge amorphous silicon is doped with phosphorus or boron,the Fermi level is believed to be moved up or down, but the density oflocalized states was so high that the dopant could not move the Fermilevel close enough to the conduction or valence bands to have aneffective p or n junction. Thus, the activation energy for the dopedglow discharge amorphous silicon was not lowered below about 0.2 eV.This result also placed a theoretical limitation on the open-circuitphotovoltage of a p-n junction of doped glow discharge amorphoussilicon, since the internal field cannot exceed the separation of theFermi level in the p and n type regions. In addition, the remainingactivation energy limits the room-temperature DC conduction of the dopedglow discharge amorphous silicon and the material would have a largesheet resistance if it were made into a large area array, the resistancenot being helped by the rather low carrier mobility which is a factor ofabout 10⁴ -10⁵ less than that for crystalline silicon. Also, where it isdesirable to dope an amorphous silicon film to form an effective ohmicinterface, for example, between an intrinsic (undoped) portion thereofand an outer metal electrode, such doped portions of the film must havea very high conductivity. The prior methods of doping such films whichdo not provide as useful an ohmic interface as in the case of the filmsof the invention to be described.

The silane glow discharge deposition of silicon films as somelimitations in addition to the less than ideal reduction of the densityof localized defect states in the energy gap achieved thereby. Forexample, such a process requires silane, which is a relatively expensivestarting material. Also, the structure of such a film and electricalcharacteristics can vary with the amount of photon absorption therein,and the film is soft and easily scratched or otherwise physicallydamaged.

The present invention provides an amorphous semiconductor material, likesilicon, with a much lower minimum density of localized defect statesthan was heretofore obtained, to provide more efficient photoconductiveand photovoltaic devices. Thus, by providing a much lower density ofdefect states, the present invention increases carrier lifetime anddepletion layer thickness. Also, it enables the amorphous silicon andother materials to be more efficiently doped by the addition of dopantmaterials thereto.

There has recently issued U.S. Pat. No. 4,196,438 to D. E. Carlson. Thispatent appears to have as its objective the making of amorphous siliconbodies by glow discharge in a gas atmosphere much less expensive thansilane. The patent states that this less expensive gas includes theelements silicon and a halogen selected from the group consisting ofchlorine, bromine and iodine. Examples of the deposition gas given aredichlorosilane (SiH₂ Cl₂), chlorosilane (SiH₃ Cl), trichlorosilane(SiHCl₃), bromosilane (SiH₃ Br), dibromosilane (SiH₂ Br₂) and silicontetrachloride (SiCl₄) with particular emphasis placed ondischlorosilane. Notably and importantly, no reference is made tofluorine. While a comparison is made in this patent between thecharacteristics of the amorphous silicon devices disclosed therein andthe prior sputtered amorphous silicon devices, just as comparisons aremade in the prior Carlson U.S. Pat. No. 4,064,521 between the prioramorphous silicon devices fabricated by glow discharge in silane andamorphous silicon devices fabricated by sputtering, there is noreference in U.S. Pat. No. 4,196,438 to any reduced density of localizeddefect states over that achieved by the use of silane, as is achieved bythe present invention. The reduction of this crucial parameter is one ofthe most important contributions of the present invention.

DESCRIPTION OF THE INVENTION

The present invention has to do with the formation of a glow dischargedeposited amorphous semiconductor body, preferably a silicon-containingfilm, containing at least fluorine as a compensating or altering agent,and most preferably at least one complementary compensating or alteringagent, such as hydrogen, both of which reduce the localized defectstates in the energy gap of the amorphous semiconductor material to adegree which either one alone could not achieve. (The term "compensatingor altering agent" refers to a material which eliminates dangling, bondsand the like. An altering agent is a compensating material which maycombine with the host matrix in amounts sufficient to form an alloy withthe host matrix). As a result, the amorphous semiconductor body providesa higher photoconductivity, wider depletion width, more efficient chargecarrier collection, longer carrier lifetime, and lower dark intrisicelectrical conductivity, where desired, and can be more easily doped toshift the Fermi level to provide very efficient n-type extrisicelectrical conductivity and the like than prior amorphous semiconductorbodies.

While the broader aspects of the invention are not to be so limited, itis believed desirable that the process condition used to introduce thefluorine and other compensating materials into the amorphoussemiconductive body are such as to chemically bond therewith in alloyingamounts which is believed further to lower the density of localizeddefect states in the energy gap. Such process conditions existinherently in the exemplary process conditions described in said U.S.Pat. No. 4,226,898 and to be described below. Thus, there is formed analloy of silicon (and other elements where desired forming the hostmatrix) and fluorine, together with any other compensating materialswhich may be used. Fluorine compensated amorphous semiconductormaterials act more like cyrstalline materials and are more useful inphotoconductive and photovoltaic devices like solar cells andphotoreceptor devices, and in current controlling devices, includng p-njunction rectifiers transistors and the like. In the case of siliconmaterials, the density of localized defect states is reduced at least bya factor of 10 or more from what was heretofore achieved, so that theminimum density of the defect states is less than 10¹⁶ per cubiccentimeter per eV.

While the broader aspects of the invention envision the compensation oralteration of an amorphous semiconductor siliconcontaining body or thelike with compensating or altering agents in a separate environment fromthe environment in which the body is formed, it is believed that thecompensation or alteration of an amorphous film formed by glow dischargedeposition equipment can be best achieved by injecting at least fluorineinto the amorphous semiconductor film more advantageously as it depositson the substrate in the glow discharge deposition process. The amountsof the compensating or altering agents including fluorine are selectedto achieve the best results for the intended use of the particular filminvolved.

In the case of silicon bodies, when a compensating or altering agent inaddition to fluorine is used, the second agent is most advantageouslyhydrogen. The fluorine and its compensating or altering agents are mostdesirably introduced in the film involved in an activated form (e.g.atomic, free radical or ionic forms thereof) and they are preferablyactivated in the vicinity of the substrate upon which the amorphoussemiconductor film is depositing, so that the one or more compensatingor altering agents reduce the localized defect states involved in themost efficient and effective manner possible. Such activated hydrogenand fluorine, for example, is produced inherently where the compensatingor altering materials are introduced in a gaseous compound in amolecular gaseous form in a glow discharge environment.

In a specific preferred embodiment of the invention, the silicon of thehost matrix is formed in the glow discharge deposition process from acompound comprising silcon tetrafluoride (SiF₄) which supplies also thefluorine as one compensating or alterant element. While fluorine couldbe added in the form of a molecular gas in its flow discharge depositionenvironment, the difficulty of handling such as corrosive gas mitigatesagainst its use in the preferred form of the invention. While silicontetrafluoride can form a plasma in a glow discharge, it is extremelydifficult to deposit silicon therefrom. However it has been discoveredthat silicon deposition takes place if the atmosphere for the glowdischarge includes preferably molecular hydrogen made reactive by theglow discharge which changes it to atomic hydrogen or hydrogen ions orthe like. This reactive hydrogen reacts in the glow discharge with thesilicon tetrafluoride so as to readily cause decomposition thereof anddeposit amorphous silicon therefrom on the substrate. At the same time,fluorine and various silicon sub fluorides are released and madereactive by the glow discharge. Also, the reactive hydrogen as well asthe reactive fluorine species are incorporated in the amorphous siliconhost matrix as it is being deposited and operate to satiate or cap thedangling bonds and other defects and in addition, alter thesemiconductor matrix and in such a way that the number of defects formedis diminished. Hence, these alterant elements reduce substantially thedensity of the localized defect states in the energy gap, with theforegoing beneficial results accruing.

In the case where fluorine and hydrogen are used to compensate or altera silicon-containing body, it is believed that the activated hydrogen iseffective in reducing the localized states in the energy gap at or nearthe Fermi level, while the activated fluorine further reduces thesestates as well as other states between those near the Fermi level andthe conduction band. The activated fluorine especially readily diffusesinto and bonds to the amorphous silicon in the matrix body,substantially to decrease the density of localized defect statestherein, because the small size of the fluorine atoms enables them to bereadily introduced into the amorphous silicon matrix. The fluorine bondsto the dangling bonds of the silicon and forms what is believed to be apartially ionic stable bond with flexible bonding angles, which resultsin a more stable and more efficient compensation or alteration than isformed by hydrogen and other compensating or altering agents. Because ofits exceedingly small size, high reactivity, specificity in chemicalbonding, and highest electronegativity, fluorine is qualitativelydifferent from other halogens and so is considered a super-halogen.These factors make fluorine and extremely efficient compensating andaltering agent.

It is believed that the silicon-containing fluorine-hydrogen alloy soformed has a lower density of defect states in the energy gap that thatachieved by the mere elimination of dangling bonds and similar defectstates. The alloying amounts of fluorine, in particular, is believed toparticipate substantially in a new structural configuration of anamorphous silicon-containing material and facilitates the addition ofother alloying materials, such as germanium. Fluorine, in addition toits other characteristics mentioned herein, is believed to be anorganizer of local structure in the silicon-contianing alloy throughinductive and ionic effects. It is believed that fluorine alsoinfluences the bonding of hydrogen by acting in a beneficial way todecrease the density of defect states which hydrogen otherwisecontributes while it is acting as a density of states reducing element.The ionic role that fluorine plays in such an alloy is believed to be animportant factor in terms of the nearest neighbor relationships.

In any amorphous semiconductor body the Fermi level cannot be movedalmost completely to the valence or conduction band needed to make agood p-n junction unless a very low density of localized states ispresent. In the attempted doping of the silane glow discharge depositionof silicon films above described with an n-conductivity dopant, theFermi level was moved to only within 0.2 eV of the conduction band. Inthe present invention, for example, the addition of an n-conductivitydopant, like arsenic, introduced in its form of arsenic gas in the glowdischarge deposition environment, to an amorphous silicon body moves theFermi level substantially all of the way to the conduction band. Theaddition of an n-dopant like arsenic to an amorphous silicon film shiftsthe Fermi level to a point near the conduction band because it isbelieved that the addition of activated hydrogen substantially reducesthe localized states at or near the Fermi level and the addition offluorine substantially reduces the density of localized defect statesbetween those at or near the Fermi level and the conduction band.Therefore, a good n-conductivity amorphous silicon film results fromsuch compensation of the body.

The production of highly efficient solar cells requires long driftlengths so that a large number of charge carriers can be separated andcollected in response to the reception of photons. This requires a largedepletion region in the amorphous semiconductor body which is a thinfilm in such an application. The comparable depletion region is obtainedin an intrinsic film when there is a low density of localized defectstates producing a low dark conductivity in the amorphous semiconductorfilm. Such semiconductor film is useful in a Schottky barrier solarcell. However, when it is desired to form a p-i-n or p-n junction solarcell, it is necessary to add dopant agents to move the Fermi levelsubstantially toward the valence and conduction bands to form effectivesolar cell p and n regions. In such case, a relatively small amount ofdopant is added to the film, so that a sufficiently wide depletionregion is maintained. As previously indicated, a low density oflocalized defect states (with an accompanying low density ofrecombination centers) enables the amorphous semiconductor film involvedto be effectively doped, thusly to form such effective p and n junctionsuseful in photo-cells. To increase the photoconductive properties ofamorphous semiconductor films further, the low density of localizeddefect states makes readily possible also the addition of sensitizingagents like zinc and copper to increase carrier lifetime. If a high darkconductivity is desired, much larger amounts of the dopant agent wouldgenerally be added to the portion of the film involved.

The principles of this invention have its most imporant utility inamorphous silicon alloy films, including silicon and other Group IVelements having normal tetrahedral bonding and three-dimensionalstability, especially germanium. Other amorphous alloys which havebonding configurations with fluorine and hydrogen or the like analogousto those existing in the alloys described above are also to beencompassed within the present invention.

Oxygen as a structural element does not have deleterious effects and maycontribute in an alloying or structural way which assures amorphicity.Carbon, as an element when properly introduced, can be an alloying agentand still be utilized with hydrogen and fluorine.

In summary, to bring the signficance of the present invention intofocus, it is believed that the present invention provides amorphoussemiconductor bodies which are electronically more like crystallinebodies than heretofore achieved for use in the manufacture ofphotoconductive and photovoltaic devices and and p-n junction currentcontrol devices and the like, despite the previously accepted dogma thatamorphous semiconductor materials could not be produced in a manner tobe similar to their crystalline counterparts. Additionally, the presentinvention provides large area high yield, low cost amorphoussemiconductor bodies which provide maximum recombination sitecompensation, charge carrier separation, and collection for solar cellsand, therefore, produce such high energy conversion efficiencies thatthey should materially contribute to the solution of the energy shortageproblems confronting the work to a greater degree each year.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic illustration of an apparatus for the glowdischarge decomposition of a compound to deposit a solid amorphous hostmatrix and to alter and dope the same.

FIG. 2 is a sectional view through a portion of the apparatusillustrated in FIG. 1 and taken substantially along the line 2--2 ofFIG. 1.

FIG. 3 is a fragmentary view of a photoresponsive device, such as foundin a Xerox drum, including an amorphous semiconductor film made inaccordance with this invention.

FIG. 4 is a fragmentary view of a photodetection device which includesan amorphous semiconductor film made in accordance with this invention.

FIG. 5 is a fragmentary view of one embodiment of a solar cell toillustrate one application of the amorphous semiconductor film made inaccordance with this invention and being directed to a Schottky barrierdevice.

FIG. 6 ia a p-i-n solar cell device which includes an amorphoussemiconductor film made in accordance with this invention.

FIG. 7 is a fragmentary view of a p-i-n junction solar cell deviceutilizing an amorphous semiconductor film made in accordance with thisinvention.

FIG. 8 is a heterojunction photovoltaic device including an amorphoussemiconductor film made in accordance with this invention.

FIG. 8A is a m-i-s solar cell utilizing an amorphous semiconductor filmmade in accordance with this invention.

FIG. 8B is a p-n-p transistor-like device utilizing an amorphoussemiconductor film made in accordance with this invention.

FIG. 9 is a graph setting forth curves plotting dark electricalconductivity, σ(Ωcm)⁻¹, against inverse temperature. 10³ /T, for variousratios of silicon tetrafluoride to hydrogen, SiF₄ /H₂, in the reactiongas for showing decrease in the electrical conductivity for increase inhydrogen content in the reaction gas.

FIG. 10 is a graph setting forth curves for various ratios of SiF₄ /H₂in the reaction gas, giving (a) room temperature dark electricalconductivity, σ_(D) (Ωcm),⁻¹ and (b) the electrical activation energy,ΔE(eV), and (c) preexponent C_(o).

FIG. 11 is a graph setting forth a curve for various ratios of SiF₄ /H₂in the reaction gas giving E₀₄ (eV), the photon energy for which theabsorption coefficient is α=10⁴ cm⁻¹.

FIG. 12 is a graph setting forth curves for various ratios of SiF₄ /H₂in the reaction gas giving the photoconductivity σ_(P) =σ_(L) -σ_(D)(Ωcm)⁻¹, where σ_(L) is the electrical conductivity under light andσ_(D) is the dark electrical conductivity, for two types of incidentradiation, where σ_(P) is measured (a) under AM-1 condition radiation,incident power of 90 mWcm⁻², and (b) under monochromatic radiation,radition at λ=600 nm with incident flux density of N_(O) =6×10¹⁴ s⁻¹cm⁻².

FIG. 13 is a graph wherein (a) the curve is a plot of incidentmonochromatic radiation, hν (eV), where h is a constant and ν isinversely proportional to the wavelength, versus the photo currentexpression, ##EQU1## and where the E_(o) intercept defines the opticalgap, and (b) the curve is a plot of the optical gap, E_(o) (eV), fordifferent ratios of SiF₄ /H₂ in the reaction gas.

FIG. 14 is a graph which plots the amount of doping of a SiF₄ +H₂ filmof this invention in the p and n directions in (a) versus darkelectrical conductivity, σ_(D), and in (b) versus electrical activationenergy, ΔE.

FIG. 15 is a graph which plots the density of localized states in theenergy gap, N(E), versus electron energy, E(eV) in the energy gap. T_(s)is the substrate temperature.

DESCRIPTION OF EXEMPLARY EMBODIMENTS OF INVENTION

The method and apparatus for depositing the semiconductor host matrix byglow discharge deposition, diagrammatically illustrated in FIGS. 1 and2, includes a housing 10 providing a vacuum chamber 11, an inlet chamber12 and an outlet chamber 13. A cathode backing member 14 is mounted inthe vacuum chamber 11 through an insulator 15 and it iscircumferentially provided with an insulating sleeve 16 and a dark spaceshield 17. A substrate 18 is secured to the inner end of the cathodebacking member 14 by a holder 19 which may be screwed onto the cathodebacking member 14 in electrical contact therewith. The cathode backingmember 14 is provided with a well receiving an electrical heater 20 forheating the same and a temperature responsive probe 21 for measuring thetemperature of the cathode backing member 14. The temperature probe 21is used in connection with the control of the energization of the heater20 to maintain the cathode backing member 14 and hence the substrate 18at desired selected temperatures.

The apparatus also includes an electrode 23 which is secured in thevacuum chamber 11 of the housing 10 in spaced relation to the cathodebacking member 14. The electrode 23 is provided with a shield 24 whichin turn carries a substrate 25. The electrode 23 is also provided with awell for receiving an electrical heater 26 and with a well receiving atemperature probe 27. The temperature probe 27 is used in connectionwith the control of the energization of the heater 26 to maintain theelectrode 23 and hence the substrate 25 at desired selectedtemperatures. The space in the vacuum chamber 11 between the cathodebacking member 14 and the electrode 23 provides for a glow dischargecondition therebetween so as to produce a plasma therebetween. Thecathode is electrically connected to a source of power comprising anR.F. or D.C. energy source which are regulatable and the electrode 23 isconnected to ground to provide the desired glow discharge therebetween.The vacuum chamber 11 is evacuated by a vacuum pump 30 through aparticle trap 31 and a pressure gauge 32 indicates the vacuum pressurein the vacuum chamber 11 which is used in connection with the control ofthe vacuum pump.

The inlet chamber 12 of the housing 10 is preferably provided with aplurality of conduits 34 for introducing materials into the housing 10to be mixed therein and to be deposited in the vacuum chamber 11 by glowdischarge decomposition between the cathode 14 and the electrode 23 onthe substrate 18 and 25. If desired, the inlet chamber 12 could belocated at a remote point for premixing the gases thereat before theyare fed into the vacuum chamber 11 of the housing 10. The materials arefed to the conduits 34 through filters or purifying devices 35 under thecontrol of valves 36. The valves 36 control the rate of admission of thematerials into the vacuum chamber 11. Where a material which is not ingaseous form but in a liquid or solid form is to be utilized, it may bearranged within a closed container 37 as indicated at 38, the material38 being heated by a heater 39 to increase the vapor pressure thereof soas to provide a vapor thereof in the container 37. A suitable gas is fedthrough a dip tube 40 into the material 38 so as to entrap vapors of thematerials 38 and convey the same through the filter or purifying device35 into its associated conduit 34 for introduction into the vacuumchamber 11. The inlet chamber 12 and the outlet chamber 13 are providedwith screens 42 to confine the plasma in the vacuum chamber 11principally between the cathode 14 and the electrode 23.

The materials fed through the conduits 34 and mixed in the inlet chamber12 are subjected to the glow discharge decomposition between the cathode14 and the electrode 23 in the vacuum chamber 11 so as to provide thedesired glow discharge decomposition and the formation of the amorphoushost matrix on the substrates 11 and/or 25 and the incorporation of thedesired alterant and/or dopant or other materials therein.

In the operation of the apparatus illustrated in FIGS. 1 and 2, thesystem is first pumped down to a pressure less than about 20 m torrprior to deposition. A gas of silicon tetrafluoride (SiF₄) is fed intothe inlet chamber 12 through one of the conduits 34 and molecularhydrogen gas (H₂) is fed into the inlet chamber 11 through another ofthe conduits 34, the two gases being premixed in the inlet chamber 12.The gas mixture may be fed at a constant SiF₄ /H₂ ratio of about 5/1into the vacuum chamber 11, the pressure of which is maintained withinthe range of about 0.3 to 2 torr. The partial pressure in the vacuumchamber 11 and the gases introduced therein provide an atmospheretherein which contains such gases. A plasma is generated in saidatmosphere between the substrates 18 and 25 using either a D. C. voltageof about 2000 volts or by radio frequency power of about 20.5 watts,operating at 13.56 MHz or other desired frequency, assuming an electrodespacing of about 1/2-1 inches and deposition area of 5-20 square inches.

While a self-sustained plasma is obtained for SiF₄, SiF₄ +Ar, SiF₄ +H₂gas mixtures, however, in our experience the deposition of a siliconfilm occurs on the substrates 18, 25 only for the last mixture includingSiF₄ +H₂. The introduction of H₂ is very important for a successfuldeposition of a film. This is due to the fact that in the plasma regionbetween the substrates 18 and 25, the hydrogen molecules H₂ aredisassociated into their atomic or ionic species. The H atoms or ionsare very reactive and possess far too much energy for directrecombination. This energy is dissipated into an inelastic collisioninvolving the SiF₄ molecules with the result that the SiF₄ moleculesdecompose into a variety of species such as atoms, sub-fluorides, freeradicals, ions, both stable and unstable of the silicon and thefluorine. This decomposition into silicon occurs in a very activeenvironment containing reactive hydrogen and reactive fluorine. Theproperties of the deposited silicon films on the substrates 18 and 25vary markedly with the ratio of SiF₄ and H₂ in the starting mixturewhich is explained in a consistent manner on the basis of inclusion of Hand F in the final deposit of the amorphous silicon.

The substrate temperature is most advantageously in the range of fromabout 200° C. to 350° C., and preferably about 250° C. The broadestaspect of the invention envisions the fluorine compensation or alteringof the depositing silicon film in the glow discharge environment at amore ideal substrate temperature for fluorine compensation oralteration, namely a temperature like about 400° C., which is in excessof that where hydrogen will remain in and compensate or alter thedepositing film. Then, in either the same or a different environment butat a much lower substrate temperature like about 200° C., hydorgen,compensation or alteration, is carried out by introducing the hydrogeninto the previously fluorine compensated or altered film by a glowdischarge or other process which enables the hydrogen readily to diffuseand bond with the fluorine compensated or altered film. (It should beunderstood that the temperatures referred to in this application aremeasured by the thermocouple placed about one-half inch from thesubstrate).

The electrical conductivity of glow discharge deposited films can bedescribed by the equation: ##EQU2## The first term of the equationdescribes the conduction of thermally generated carriers into theextended states above E_(c) (the conduction band) or below E_(v) (thevalence band). The second term of the equation represents conduction byhopping within the localized defect states of the energy band and thisis predominant when the density of localized defect states is large, aswith unaltered silicon or the like. The pre-exponent in the first termof the aforementioned equation is represented by:

σ_(o) =e/μ_(c) N (E_(c)) kT exp (σ/k) for electrons

σ_(o) =e/μ_(h) N (E_(v)) kT exp (σ/k) for holes

where N (E_(c)) and N (E_(v)) are the effective densities of the statesat the conduction band mobility edge E_(c) and of the valence bandmobility edge E_(v), respectively. μ_(e) and μ_(h) are the mobilities ofelectrons and holes, respectively, in the extended states. σ describesthe temperature dependence of the band edges and k is a constant.

FIG. 9 shows a series of curves for log σ vs. 10³ /T for several samplesincluding the designated ratios of SiF₄ /H₂ in the reaction gas, namely,99:1, 80:1, 30:1 and 15:1. It is clear that as the H₂ content of the gasmixture is increased, the conduction mechanism in the films changes froman unactivated process involving hopping to a well defined activatedprocess involving thermally generated carriers. The samples having theratios 99:1 and 80:1 can be described by the second term in theaforementioned equation involving hopping and the samples involving theratios of 30:1 and 15:1 can be described by the first term of theaforementioned equation involving thermally generated charge carriers.It is thus seen that the dark electrical conductivity at roomtemperature of various samples decreases substnatially as the H contentin the samples increases.

FIG. 10 summarizes the parameters (a) room temperature dark conductivityσ_(D), (b) the activation energy, ΔE, and (c) the preexponent σ_(o). Theactivation energy for samples having the ratios 99:1 and 80:1 asindicated in (b) are unactivated, as discussed above, while the othersamples having the ratios 30:1 and 15:1 are activated. In order toobtain ΔE and σ_(o) for SiF₄ /H₂ greater than 80:1, tangents to thecurves therefor in FIG. 9 at room temperature were drawn. This figureshows clearly that a transition in the conduction mechanisms occurs inthe range 30/1<SiF₄ /H₂ <80/1. The factor σ_(o) as shown in (c) changesby six orders of magnitude. The room temperature dark conductivity σ_(D)decreases markedly as the H₂ content in the mixture increases. A minimumin the electrical conductivity σ_(D) occurs at a ratio SiF₄ /H₂ at about15:1 and the electrical conductivity σ_(D) increases with furtherreduction in the ratio.

FIG. 11 shows that the energy or band gap E₀₄ (defined as the photonenergy at which the absorption coefficient α=10⁴ cm⁻¹ increases as theratio SiF₄ /H₂ is decreased. There is a correlation between the ratio ofSiF₄ /H₂ in the gas mixture and the ratio of Si/H and the ratio of Si/Fin the deposited film although, these correlations may not be aone-to-one correlation. When alloys are formed, the E₀₄ increases sincehydrogen is incorporated into the film increasingly when the ratio SiF₄/H₂ is decreased. This results in an increase in the number of Si-Hbonds and since these bonds are stronger than the Si-Si bonds, E₀₄ istherefor increased. The increase in the incorporation of hydrogen withinthe deposited film reflects in changes in the optical and electricalproperties as is presented in FIGS. 9 to 11.

The least hydrogenated samples (99:1 and 80:1) exhibit an unactivatedconduction, the conduction being n-type and yielding a localized statedensity greater than 10¹⁹ cm⁻³ (eV)⁻¹. The conduction of such samples isdominated by hopping of electrons situated around the Fermi level. Asthe ratio SiF₄ /H₂ is decreased, it is noted from FIGS. 9 and 10 thatthe electrical conductivity decreases by orders of magnitude and thechange to a well-defined activation energy takes place. These samplesremain n-type and the conduction mechanism is electron conduction withinthe extended states at the conduction band. The above transition from ahopping conduction to a well activated conduction is due to hydrogen andfluorine incorporation into the amorphous silicon film. The localizedstates originate from a variety of source, such as dangling bonds,voids, defects, etc. with increased hydrogen inclusion, the danglingbonds are eliminated to alter the semiconductor matrix in such a waythat the density of localized defect states is reduced. When asufficient reduction takes place, a transition in the conductionmechanism, of the above type, occurs.

It is generally known that in the weakly absorbing part of the energyspectrum, such that α d is less than 1 where α is the absorptioncoefficient (cm⁻¹) and d (microns) is the thickness of film, thephotocurrent in the film can be expressed in the following way, ##EQU3##where Ip=photocurrent, h=Plancks constant, γ=inverse proportional to thewave length of the incident radiation, N_(o) =incident flux density ofthe radiation, R=reflection coefficient, e=electronic charge and E_(o)=optical gap. FIG. 13(a) shows a plot of these parameters. Extropolatingthe well-defined straight region leads to an intercept on the h γ axis,which yields the optical gap, E_(o). FIG. 13(b) shows the variation ofE_(o) with SiF₄ /H₂. There is a general increase in optical gap, E_(o),with increasing hydrogen incorporation and agrees well with E_(o4)variation with variation of the SiF₄ /H₂ ratio. The highlyphotoconducting films occur when the ratio SiF₄ /H₂ is about 10:1. Thisis shown in FIG. 12(a) and (b) for two types of incident radiation whereσ_(P) is plotted as a function of SiF₄ /H₂ gas ratio. When these filmswere subjected to prolonged AM-1 radiation, no photostructural changewith respect to its properties was noted.

As expressed above, the glow discharge deposition of the amorphoussilicon film from silicon tetrafluoride in an atomosphere containinghydrogen, wherein the hydrogen and fluorine are made reactive by theglow discharge and are incorporated into the film for altering the same,gives drastically improved results over silicon films formed by glowdischarge of silane and other hydrogen as mentioned in carbon U.S. Pat.No. 4,196,438. This is graphically illustrated in FIG. 15 wherein thedensity of states, N(E) cm⁻³ eV⁻¹, as a function of electron energyE(eV) between the conduction band E_(c) and the valence band E_(V) isplotted for the two films. The solid curve shows the film deposited fromsilane (SiH₄) at a substrate temperature of about 250° C. and the dottedcurve shows the film deposited from silicon tetrafluoride (SiF₄) in anatmosphere containing hydrogen H₂ having SiF₄ /_(gas) ratio of 10/1 at asubstrate temperature of about 250° C. The SiF₄ /H₂ gas ratio mostadvantageously is no greater than 14/1, and not less than about 2/1, andis preferably about 5/1.

It is first noted that the density of localized states of the SiF₄ /H₂film is much less than that of the SiH₄ film, the former being in therange of 10¹⁶ cm⁻³ eV⁻¹ and the latter being in the range of 10¹⁷ cm⁻³eV⁻¹. These densities of states are determined by the field effecttechnique and these numbers represent upper limits. It is also noted inFIG. 15 that the SiH₄ film has a hump or bump at substantially 0.4 eV ofthe energy gap which hinders by subsequent doping the shifting of theFermi level by doping much beyond this point and not beyond an energygap value of 0.2 eV. However, in the SiF₄ /H₂ film of this invention,the bump is completely eliminated so that the Fermi level can be shiftedsubstantially to the conduction band E_(C). This is due to the combineduse of the hydrogen and the fluorine as compensating or alterantelements in the semiconductor film of this invention and is an importantfactor of this invention. It is, therefore, possible in the SiF₄ /H₂produced film to readily modify or dope the film to shift the Fermilevel substantially as desired.

In the SiH₄ produced film, there is also a hump at the 1.2 eV value inthe energy gap which prevents the shifting of the Fermi level muchtherebeyond toward the valence band E_(V). By utilizing othercompensating or alterant elements during the glow discharge depositionof the SiF₄ /H₂ film a curve corresponding to the dotted curve extendingtoward the valence band could also be provided. This further reductionof localized defect states in the gap near the conduction band and thevalence band make the amorphous semiconductor film of this invention thesubstantial equivalent of crystalline semiconductors so that theamorphous semiconductor material of this invention may be readily dopedas in crystalline semiconductors to obtain all of the favorableattributes thereof.

As previously indicated, to produce useful amorphous silicon films whichhave the desired characteristics for use in solar cells, p-n junctioncurrent control devices, etc., the compensating or altering agentsproduce a very low density of localized defect states in the energy gapwithout changing the basic intrinsic character of this film.Compensation of dangling bonds and the like is achieved with theaddition to the amorphous films of hydrogen and fluorine of far lessthan one atomic percentage of the amorphous composition involved. Theamount of fluorine and hydrogen most desirably used forms asilicon-fluorine-hydrogen alloy, where both the hydrogen and fluorinemay constitute well in excess of even one atomic percentage of thealloy, such as about 3 atomic percent and as much as about 10 percent.The amount of fluorine is preferred to be about 3 atomic percent and theamount of hydrogen is preferred to be about 5 atomic percent of thealloy.

The temperature of the substrate is adjusted to obtain the maximumreduction in the density of the localized defect states in the energygap of the amorphous semiconductor film involved. The substrate surfacetemperature will generally be such that it ensures high mobility ofdepositing materials, and one below the crystallization temperature ofthe depositing film.

In this connection, reference is made to FIG. 14, wherein doping of thesemiconductor material of this invention with arsene (AsH₃) towardn-type conductivity and with diborane (B₂ H₆) toward the p-typeconductivity are illustrated. In 14(a) the undoped semiconductor of thisinvention has an electrical conductivity of about 10⁻⁷ (Ωcm)⁻¹ and inFIG. 14(b) it has an electrical activation energy of about 0.6 eV. Whenthe semiconductor material is doped with arsine in small amounts, partsper million, the dark electrical conductivity increases substantiallywhich establishes that the Fermi level may be readily and easily shiftedto the conduction band with a minimal amount of modification or doping.Also, as shown in FIG. 14(b) the electrical activation energy Δ E issubstantially reduced by the use of small amounts of such dopant.

Where, however, diborane is used as the dopant toward p-typeconductivity relatively large amounts of dopant material are required.This p-type dopant first acts to decrease the dark electricalconductivity and to increase the electrical activation energy to a pointwhere the ratio of the diborane to the SiF₄ +H₂ amounts to about 10⁻⁴.At the same time, the electrical activation energy is increased.Thereafter, further increases in the ratio operate to decrease theelectrical conductivity and decrease the electrical activation energy asseen from FIG. 14. Considerably more doping toward the p-typeconductivity is required, this being due to the density of defect statesnear the valence band. As expressly above, the amount of doping towardthe p-type conductivity, may be decreased substantially by utilizing afurther compensating or alterant material for decreasing the density ofdefect states near the valence band. In addition to utilizing diborane(B₂ H₆) for p-type doping, for example, Ga(CH₃) or (C₂ H₅), Al, or thelike may be utilized. Here, the gallium and the aluminum included in thefilm are obtained from these gases.

Thus, in accordance with this invention, the use of SiF₄ /H₂ as thegases to be decomposed by glow discharge, in addition to decreasing thedensity of defect states in the energy gap also provide for readyshifting of the Fermi level with small amounts of dopants at least tothe conduction band. The dopants arsine and diborane are suppliedthrough appropriate conduits 34 into the inlet chamber 12 where they maybe premixed into the SiF₄ +H₂ gas so that they are incorporated in theamorphous silicon film as the same is being glow discharge deposited inthe vacuum chamber 11. The glow discharge breaks up these gases into avariety of species, such as atoms, free radical, ions, both stable andunstable including arsenic from the arsine or boron from the diborane,which are incorporated as dopants into the amorphous film while the sameis being deposited.

Due to the nature of the amorphous semiconductor films of thisinvention, which substantially simulate crystalline semiconductors,having relatively low dark electrical conductivity, relatively highlight electrical conductivity or photoconductivity, ease of doping for nand p-type conduction and shifting of the Fermi level to the conductionand valence bands, and increased carrier drift mobility, including holemobility as well as electron mobility, it is possible, for the firsttime, to make commercially acceptable amorphous semiconductor devices.Examples of some of such devices are diagrammatically illustrated inFIGS. 3 to 8.

FIG. 3 diagrammatically illustrates a fragment of a photo-responsivedevice such as found on a Xerox drum, including a amorphoussemiconductor film 51 compensated or altered in the manner previouslydescribed which is deposited on a metal substrtate 50. The metalsubstrate 50 is highly conductive and selected to provide ohmic contact52 with the amorphous semiconductor film 51. The amorphous semiconductormaterial in this example is of the n-type conductivity. Preferably indepositing the film 51 on the substrate 50 it is deposited n⁺ asindicated at 51' so as to provide an n to n⁺ junction 52' to assure alow ohmic contact between the film 51 and the substrate 50. In Xeroxuse, the film 51 is normally charged and the film of the inventionretains its charge because of the relatively low dark electricalconductivity thereof. When, however, the film is imagewise subjected tolight radiation, the electrical conductivity thereof is increased whereso radiated so as to release the charge thereat and this is madepossible by the relatively high light electrical conductivity orphotoconductivity of this invention. The differences in the charged anduncharged portions of the film control a toner applied to the film forproviding suitable images in a xerographic process.

FIG. 4 diagrammatically illustrates a photodetection device whichincludes an amorphous semiconductor film 51 compensated or altered inthe manner previously described and applied or deposited on a metalsubstrate 50 at 52. A light transmitting metal film 53, is applied tothe semiconductor film 51 to form an electrode therefor. The film 53 hasgood electrical conductivity as does the substrate 50. Electrodes 55 areprovided for applying a voltage across the film 53 and substrate 50which is normally blocked by reason of the relatively low darkelectrical conductivity of the film 51. An anti-reflection layer 56 maybe applied over the film 53 and electrodes 55. When incident radiationis applied to the device of FIG. 4, the device will conduct currentbecause of the relatively high light electrical conductivity orphotoconductivity of the film of the invention. Thus, a current in acircuit may be controlled in accordance with the amount of incidentradiation applied to the device. In order to assure ohmic contactbetween the film 51 which has n-type conductivity and the metal film 53and substrate 50, the film 51 is doped n⁺ at 51', the surfaces betweenthe film 51 and the n⁺ doped portions 51' being indicated at 52' and54'.

FIG. 5 is a diagrammatic illustration of a Schottky barrier type solarcell. It includes a metallic substrate 50 on which is deposited at 52 asemiconductor film 51 compensated or altered in the manner previouslydescribed. In order to insure good ohmic contact between thesemiconductor film 51 and the metal substrate 50, the film 51 is highlydoped n⁺ as indicated at 51', the juncture of the film 51 and the highlydoped portion 51' thereof being indicated at 52'. A metallic film 58 isdeposited at 59 on the semiconductor film 51 and the metallic film 58 istransparent to solar radiation and is of a metallic material with goodelectrical conductivity and of a high work function, as for example,platinum, palladium, chromium, iridium or rhodium. On the surface of themetallic film 58 is a grid type electrode 60 having good electricalconductivity. The function of the grid electrode 60 is for the uniformcollection of current from the metallic layer 58. An antireflectionlayer 61 which is transparent is deposited over the metallic film 58 andelectrode 60.

A Schottky barrier is formed at the interface 59 by contacting themetallic film 58 with the amorphous film 51. The Schottky barriergenerates a space charge region in the semiconductor material 51 whichpenetrates into the same from the interface. The space charge region isalso referred to a the depletion region and preferably the depletionregion extends the entire width of the semiconductor film 51. Carrierscreated anywhere in the film 51 as a result of the absorption of solarradiation are swept by the electric field in the depletion region toenter the metallic substrate 50 or the metallic film 58. In this way, aphotovoltaic current is produced which can be introudced into a circuitconnected to the metallic substrate 50 and the grid type electrode 60.

A photovoltaic device and more particularly a p-i-n solar cell isdiagrammatically illustrated in FIG. 6. It includes a semiconductor film66 compensated or altered in the manner previously described depositedon a metallic substrate 65. The semiconductor film 66 includes a portion67 which is doped to be n-type, a central portion 68 which isessentially intrinsic although it is slightly n-type and doped p-typeportion 69. A transparent metallic film 70 is deposited over thesemiconductor film 66, the substrate 65 and the film 70 operating aselectrodes for conducting current generated by the photovoltaic device.In order to provide an ohmic contact between the metallic substrate 65and the n doped portion 67, the portion 67 is highly doped n⁺ asindicated at 67'. Likewise, in order to provide ohmic contact with themetallic film 70, the p doped portion 69 of the semiconductor film ishighly doped p⁺ as indicated at 69'.

In such a p-i-n solar cell, as a result of the equalization in Fermilevels between portions 69, 67 and 68 there is a negative space chargein the portion 69 and a positive space charge in the portion 67 and theformation of built-in potential between the portions 69 and 68 and and67 and 68, and also the formation of a depletion region in the intrinsicportion 68. Preferably, the depletion region extends through theintrinsic portion 68 and, therefore, any carriers generated in theintrinsic portion 68 by the absorption of solar radiation will be sweptup in the electrical field of the depletion region and collected as anelectrical current.

A p-n junction solar cell device is illustrated in FIG. 7. Thisphotovoltaic device includes a metallic substrate 75 and an amorphoussemiconductor material 76 compensated or altered in the mannerpreviously described. The semiconductor film 76 includes a portion 77which contacts the metal substrate 75 and which is highly doped n⁺ tomake ohmic contact at 78 with the portion 79 which is doped n-type. Thefilm also includes a portion 80 which is doped p-type to provide a p-njunction 81 between the portions 79 and 80. The p-type doped layer 80 isfurther highly doped p⁺ as indicated at 80' so as to provide an ohmiccontact with a transparent metallic film 83 contacting the semiconductorfilm 76. In the operation of this photovoltaic device, solar radiationenters the device through the transparent metallic film 83 and some ofthe solar radiation is absorbed in the semiconductor film 76 formingelectron-hole pairs. These current carriers then diffuse to the p-njunction 81 and if they arrive at the space charge region of the p-njunction, before recombining they are collected and contribute to thecurrent generated by the device.

FIG. 8 is a diagrammatic illustration of a heterojunction photovoltaicdevice including an amorphous semiconductor film 88 compensated oraltered in the manner previously described. The film has n-typeconductivity and it is provided with a highly doped n⁺ portion 86 whereit contacts a metal substrate 85 so as to provide An ohmic contact 87for the amorphous film 88. The device also includes a semiconductor film89 joining the film 88 at 90. The film 89 may be of a different materialthan the film 88 and has a high band gap so as to be transparent tosolar radiation impinging the same. The differences in band gap betweenthe films 88 and 89 provide a large potential barrier in the vicinity ofthe junction 90, which forms a heterojunction, for the generation of alarge open circuit voltage. The differences in the band gap produce bandbending at the heterojunction 90. The semiconductor material 89 isheavily doped so as to provide substantially ohmic contact withelectrodes 91. Because the semiconductor film 89 is highly doped, theelectrical field generated by the potential barrier at theheterojunction 90 will not penetrate substantially into the film 89 butwill penetrate substantially into the film 88. Accordingly, most of thespace charge region, i.e. the depletion region, will be generated in thesemiconductor films 88. Solar radiation which is absorbed within thedepletion region in the semiconductor film 88 will generateelectron-hole carriers which will be swept by the electrical field toeither the electrode 91 or the substrate 85, thereby generating theelectric current of the device.

FIG. 8A shows a m-i-s solar cell in fragmentary crosssection. The solarcell illustrated includes a substrate 92, previously described of amaterial having both good electrical conductivity properties and theability of making an ohmic contact with an amorphous silicon film 94'compensated or altered with at least fluorine and also preferablyhydrogen.

The film shown has an ohmic contact-providing region 94a next to thesubstrate 92, the region 94a' being shown as being a heavily dopedn-plus region which forms a low resistance interface between thesubstrate 92' and an intrinsic region 94b'. Upon the amorphous film 94'is a thin solar energy transparent insulating layer which, for example,may be made of a titanium dioxide or silicon dioxide. Upon theinsulating layer 95' is deposited a high-work function metal barrierfilm 97' like the barrier-forming layer in the Schottky barrier devicedescribed. Deposited upon the barrier-forming layer 97' are a gridelectrode 98' and a antireflection layer 96', like the correspondinggrid and layer 86 and 84 of FIG. 6.

Reference should now be made to FIG. 8B which slows a p-n-p currentcontrol device having outer electrodes 102 and 104 on the opposite facesof a film 105 of an amorphous semiconductor material like silicon,compensated or altered in accordance with the invention. The amorphoussemiconductor film 105 has outermost p-doped regions 105a and 105a and apreferably doped base-forming intermediate region 105b of n-conductivitytype. If the device is to form a transistor, a terminal connection ismade to the base-forming region 105b of the film.

The thickness of the layers of the various devices shown in FIGS. 3through 8B may vary widely. However, typical thicknesses for the layerforming regions of the amorphous films involved are of the order ofmagnitude of, for example, about 5000 Angstroms. The various adjacentregions of the films have thicknesses of the order of magnitude of about100 to 200 Angstroms. Typical thicknesses of the anti-reflection layersis about 800 Angstroms. The insulating layer in the m-i-s devicedescribed is of the order of magnitude of about 20 Angstroms.

In the formation of devices, such as in FIGS. 3 to 8B the composition ofthe gases being decomposed can be rapidly changed such that any desiredsequence of layers with different characteristics, such as,substantially intrinsic, p-type and n-type, p⁺ -type and n⁺ -type, canbe fabricated by the glow discharge process of this invention.

As expressed above, other compounds than silicon tetrafluoride glowdischarge decomposed in an atmosphere of hydrogen can be utilized inaccordance with this invention. As one example, silane (SiH₄) can bedecomposed in an atmosphere containing hydrogen to provide a substantialdecrease in the density of states in the energy gap over that obtainedmerely by the glow discharge decomposition of silane.

Where the glow discharge decomposition of a compound containing siliconand involving different compensation or alterant elements, such ashydrogen and fluorine are involved, in addition to utilizing silicontetrafluoride in an atmosphere containing hydrogen (SiF₄ +H₂), othercompounds and atmospheres may be utilized, as for example,

SiH₄ +SiF₄

SiH₄ +F₂

SiHF₃ +H₂

SiH₂ F₂ +H₂

SiHF₃ F+H₂

where hydrogen and fluorine form the compensation or alterant elementsincorporated into the amorphous semiconductor silicon host matrix.

Other examples of compounds and elements in the atmosphere include:

SiHCl₃ +F₂

SiH₂ Cl₂ +F₂

SiH₃ Cl+F₂

SiCl₃ F+H₂

SiCl₂ F₂ +H₂

SiClF₃ +H₂

wherein hydrogen and fluorine are compensation or alterant elementsincorporated in the amorphous silicon host matrix.

Instead of utilizing only the Group IV element silicon (Si) anotherGroup IV element may be ultilized with silicon or instead of siliconsuch as germanium (Ge), by glow discharge decomposition including thefollowing compounds in the following atmosphere:

    GeH.sub.4 +F.sub.2

    GeF.sub.4 +H.sub.2

the hydrogen, fluorine and fluorine being therein for compensation oralteration purposes.

In summary, to bring the significance of the present invention intofocus, it is believed that the present invention enables the fabricationof amorphous semiconductor films which are more similar to crystallinefilms for use in the manufacture of solar cells and current controldevices including p-n junctions and the like, despite the previouslyaccepted dogma that amorphous semiconductor materials could not beproduced in a manner to be equivalent to their crystalline counterparts.Additionally, the present invention provides large area, high yield, lowcost amorphous semiconductor films. Finally, the films produced so as toprovide maximum charge carrier separation and collection for solar cellsproduce such high energy conversion efficiencies that they shouldmaterially contribute to the solution of the energy shortage problemsconfronting the world to a greater degree each year.

It is understood that numerous modifications may be made in the variouspreferred forms of the invention described, without deviating from thebroader aspects thereof.

In the claims, when a reference is made to a "film", such expression isintended to cover a layer of material formed in a single process step ora multiplicity of layers of or similar material which may be formedsequentially in the same or different environments.

We claim:
 1. A photodetection device comprising: means responsive to theintensity of incident radiation for providing a corresponding outputsignal, said responsive means including a thin semiconductor filmcomprising a solid amorphous silicon or amorphous germanium host matrixhaving electronic configurations with an energy gap; said host matrix ofthe amorphous semiconductor material further including at least onecompensating element for reducing the density of localized states insaid energy gap.
 2. A device as in claim 1 wherein said compensatingelements are chosen from the group consisting essentially of hydrogen,fluorine, and combinations thereof.
 3. A photoresponsive device as inclaim 1 wherein said thin film alloy material is adapted to generateelectron-hole pairs in response to the absorption for incident radiationso as to provide for a flow of electrical current corresponding to theintensity of said radiation.
 4. A photoresponsive device as in claim 3further including a circuit in which current may be controlled inaccordance with the intensity of the incident radiation.
 5. A device asin claim 1 further including means adapted to provide a voltage acrossthe thin film alloy material.
 6. A device as in claim 5, wherein saidvoltage providing means include at least one electrode electricallycommunicating with said semiconductor film.
 7. A device as in claim 6,wherein at least one of said electrodes is light transmitting.
 8. Adevice as in claim 1, further including an anti-reflective film on thelight incident side of the thin film material.
 9. A device as in claim8, further including a heavily doped semiconductor alloy materialdisposed between the film semiconductor and the electrode opposite thelight incident electrode.
 10. A device as in claim 9, wherein saidheavily doped material is N+material.
 11. A device as in claim 1,further including means adapted to provide for a flow of electriccurrent through the thin film alloy material.
 12. A device as in claim1, wherein said thin film material is of low dark electricalconductivity.
 13. A device as in claim 1, wherein said thin filmmaterial is of high light electrical conductivity.
 14. A device as inclaim 1, wherein the semiconductor film is a p-n diode.
 15. A device asin claim 1, wherein the semiconductor film is a p-i-n diode.
 16. Adevice as in claim 1, wherein the semiconductor film is part of aSchottky barrier.
 17. A current control device comprising meansresponsive to the intensity of incident radiation for providing acorresponding output signal, said responsive means including a thinsemiconductor film comprising a solid amorphous silicon or amorphousgermanium host matrix having electronic configurations with an energygap; said host matrix of the amorphous semiconductor material furtherincluding at least one compensating element for reducing the density oflocalized states in said energy gap.
 18. A device as in claim 17,wherein the semiconductor film is a p-n diode.
 19. A device as in claim17, wherein the semiconductor film is a p-i-n diode.
 20. A device as inclaim 17, wherein the semiconductor film is part of a Schottky barrier.